In many conventional four cycle engines each piston is individually connected to the drive shaft by means of a connecting rod and crank arm. The drive shaft comprises a crankshaft having one crank pin and counterweight for each piston.
The conventional crankshaft is relatively long and heavy, especially in the case of eight cylinder in-line engines. In order to balance the internal engine forces, several relatively heavy counterweights are required (one for each piston). The crankshaft cost becomes a major factor. Also, the crankshaft bearings have to be relatively heavy and numerous to absorb momentary unbalanced forces.
Another problem with conventional piston engines is that each piston lacks lateral support. The conventional connecting rod obliquely transmits the axial force component of the piston without restraining the piston against lateral movement, such that the piston exerts a considerable lateral force on the cylinder wall. The piston has to be relatively long to distribute the lateral forces and prevent excessive wear on the piston or cylinder wall.
The long piston requirement and crankshaft design requirements tend to unduly increase the overall size and weight of the engine, especially with engines having a large number of cylinders.
Some engine designs have been proposed to overcome problems associated with conventional piston engines. In one such engine design the pistons are aligned in pairs. Two opposed in-line pistons are rigidly connected together for conjoint movement by a toothed rack, so that one piston moves toward the top dead center position while the other piston moves away from the dead center position and vice versa. The pistons can be relatively short because the forces are largely axial (not lateral).
A toothed gear in mesh with the tooth rack oscillates rotationally to provide the engine output force. Special clutches and counter shafts are required to translate gear oscillational motion into one way rotation of the output shaft.
The described engine designs overcome some problems associated with conventional engines. However, such designs have their own problems, associated with the require-ment for an increased number of shafts, and gears. In most cases slip clutches are required to translate gear oscillation into one way rotation of the output shaft. Slip clutch arrangements are shown in U.S. Pat. No. 5,673,665 (Kim), and U.S. Pat. No. 5,562,075 (Walsh)
U.S. Pat. No. 5,673,665, issued to Min-Tac-Kim on Oct. 7, 1997 shows an engine that includes two opposed in-line piston-cylinder assemblies having a piston rod rigidly connecting the two pistons, whereby one piston moves toward the top dead center position while the other piston moves away from the top dead center position, and vice versa Rack gear teeth on the piston rod are in mesh with gear teeth on two counter shafts extending transverse to the motion path of the piston rod. One way clutches on the counter shafts intermittently transmit drive forces to aligned shafts that have geared connections to an output shaft located midway between the aligned shafts.
As the piston rod moves back and forth the one-way clutches are alternately in the drive mode and slip mode, so that the output shaft is driven in one direction.
The drive system described in U.S. Pat. No. 5,673,665 is relatively complex. Five separate shafts are required to produce rotary movement of the output shaft.
U.S. Pat. No. 5,562,075, issued to N. Walsh on Oct. 8, 1996, shows an engine whereon two oppositely-moving pistons are linked to a rotary shaft that rocks back and forth in synchronism with the pistons. The shaft has ratchet connections with two separate bevel gears that are in mesh with a third output gear. The bevel gears are alternately in the drive mode and slip mode, so that the output gear is driven in one direction. In many respects, the engine of U.S. Pat. No. 5,562,075 is similar to the engines of U.S. Pat. No. 5,673,665. In both cases the drive force is directed through slip clutches.
U.S. Pat. No. 5,934,243, issued to G. Kopystanski on Aug. 10, 1999, shows an engine wherein each piston has a piston rod that has one toothed rack in mesh with a power drive gear and a second toothed rack in mesh with an idler gear. Apparently each piston drives the associated power drive gear on the downstroke and the idler gear on the upstroke. A system of timing gears is apparently used to provide power to an output shaft 78 when the piston is on the upstroke. Slip clutches are used to achieve uni-directional movement of the output shaft. The drive system is quite complex. Several shafts 38, 42, 14, and 78 are required.